1. Field of the Invention
The present invention relates to a method for manufacturing a wiring circuit board which has bumps of a constant fixed height.
2. Description of the Related Art
Very small bumps (e.g., 50 xcexcm in diameter and 30 xcexcm in height) are widely used in connections between electronic component elements such as semiconductor or LCD elements and wiring circuit boards, or between the different layers in multilayered wiring substrates.
A representative method for manufacturing bumps of such size is given in FIGS. 4A to 4E.
In other words, first, a dual-layer flexible substrate 43 comprising a copper foil 42 affixed to a polyimide film 41 is prepared, as shown in FIG. 4A, after which the copper foil 42 is patterned using a photolithography technique to form a wiring circuit 44 (FIG. 4B).
Next, a cover coat layer 45 is formed on the wiring circuit 44 in accordance with a conventional method (FIG. 4C). For example, it is possible to form the cover coat layer 45 by forming a polyamic acid layer on the wiring circuit 44, patterning it using photolithography and imidising it. A resist ink can also be printed thereon.
Next, very small bumps can be formed by irradiating with a laser light the region of the polyimide film 41 which corresponds to the wiring circuit 44 to form bump holes 46 (FIG. 4D), and then, using an electrolytic plating method to form metal bumps 47 on the wiring circuit 44, which is exposed in the bottom portion of the bump holes 46. The cover coat layer 45 may be optionally covered with a protective film (not shown) after the irradiation and before the electrolytic plating (FIG. 4E).
However, when the bump holes 46 are formed by the irradiation with the laser light, the surface area of the openings will vary, owing to scattering in the amount of smearing attached to the bottom portion of the bump holes 46; therefore a drawback arises in that substantial scattering appears in the height of the metal bumps 47. Accordingly, stable bump connections become difficult to achieve. Connecting semiconductor elements to the wiring circuit in one step using ultrasonic waves becomes particularly difficult. Plating pre-treatments become essential, moreover, in order to improve the adhesive strength between the wiring circuit 44 and the metal bumps 47 formed thereon.
In view of the foregoing problems of prior art, it is an object of the present invention to provide a method for manufacturing a wiring circuit board with bumps, which enables stable bump connections and furthermore does not require complex operations such as plating pre-treatments.
The inventors have perfected the present invention based on the discovery that stable bump connections can be effected by half-etching a metal foil, which is of a thickness that is the sum of the thickness corresponding to the height of the metal bumps and the thickness of the wiring circuit layer, to a depth corresponding to the thickness of the metal bumps, it is possible to fabricate bumps of uniform height, without requiring complex operations such as plating pre-treatments.
The present invention provides a method for the manufacture of a wiring circuit board having a wiring circuit and bumps formed thereon, comprising the steps of (a) laminating a protective film onto one surface of a metal foil on which bumps are to be formed, which is of a thickness that is the sum of the thickness of the wiring circuit and the height of bumps which are to be formed on the wiring circuit and forming a wiring circuit formation etching mask on the other surface of the metal foil where the wiring circuit is to be formed; (b) forming the wiring circuit to a desired thickness by half-etching the metal foil from the wiring circuit-formation etching mask side; (c) providing a cover coat layer on the wiring circuit, after removing the wiring circuit-formation etching mask; (d) forming a bump-formation etching mask on the bump formation surface, after removing the protective film which has been provided on the bump formation surface of the metal foil; (e) forming bumps to a desired height by half-etching the metal foil from the bump-formation etching mask side; (f) forming a polyimide precursor layer so as to bury the bumps, after removing the bump-formation etching mask; and (g) forming an insulating later of a desired thickness by etching back and then imidising the polyimide precursor layer. The wiring circuit can be formed before the bumps are formed, by using this method of manufacture.
Moreover, the present invention provides a method for the manufacture of a wiring circuit board having a wiring circuit and bumps formed thereon, comprising the steps of: (aa) laminating a protective film onto one surface of a metal foil on which a wiring circuit is to be formed, which is of a thickness that is the sum of the thickness of the wiring circuit and the height of bumps which are to be formed on the wiring circuit and forming a bump-formation etching mask on the other surface of the metal foil where the bumps are to be formed; (bb) forming the bumps to a desired height by half-etching the metal foil from the bump-formation etching mask side; (cc) forming a polyimide precursor layer so as to bury the bumps, after the bump-formation etching mask has been removed; (dd) forming an insulating layer of a desired thickness by etching and then imidising the polyimide precursor layer; (ee) forming a wiring circuit formation etching mask on the wiring circuit formation surface, after removing the protective film which has been provided on the wiring circuit formation surface of the metal foil; (ff) forming a wiring circuit to a desired thickness by half-etching the metal foil from the wiring circuit-formation etching mask side; and (gg) providing a cover coat layer on the wiring circuit, after removing the wiring circuit formation etching mask. Bumps can be formed before the wiring circuits are formed, by using this method of manufacture.
Moreover, the present invention provides a method for forming bumps, comprising forming a bump-formation etching mask on the bump formation surface of a metal foil of a thickness that is the sum of the thickness of the wiring circuit and the height of the bumps which are to be formed on the wiring circuit, and half-etching the metal foil to a depth corresponding to a desired bump height from the bump-formation etching mask side.
Furthermore, the present invention provides a wiring circuit board, comprising a wiring circuit, a cover coat layer formed on one surface of the wiring circuit, an insulating layer formed on the other surface of the wiring circuit, and bumps which are connected to the wiring circuit and formed so as to protrude from the insulating layer, wherein the wiring circuit and the bumps are formed from one metal foil. It is preferable to form a contact hole in the cover coat layer to enable electric connection to the wiring circuit from the cover coat layer side. The insulating layer is preferable to be a polyimide layer obtained by imidising a polyimide precursor.